The past two decades have witnessed a dramatic rise of obesity prevalence and a parallel leap of type 2 diabetes (T2D) prevalence. Although there are clear genetic determinants for both conditions, solid evidence proves that both conditions can also be prevented through lifestyle and dietary intervention on modifiable risk factors. Meanwhile, evidence now suggests that some environmental pollutants, especially the persistent organic pollutants (POPs), may also lead to increased risk of both obesity and T2D. Accumulating data from animal studies have linked a few POPs, such as polychlorinated biphenyls (PCBs) and p,p'- dichlorodiphenyldichloethylene (p,p'-DDE), with weight gain and insulin resistance. Human studies are rare in this field and most of the existing studies used a cross-sectional design, which cannot establish causal relationship or timeline between exposures and outcomes. Moreover, our preliminary data from epidemiologic observation and system biology simulation have suggested that hexachlorobenzene (HCB), a POP primarily deemed to be merely a marker of long-term total POP exposure, may be causally associated with risk of diabetes. In addition, new POPs that have entered the environment, such as the perfluorinated compounds (PFCs), have received little attention with respect to their effects on obesity and T2D. To narrow the gap in science in this regard, we aim to evaluate PCBs, DDE, HCB, and PFCs in relation to 1) subsequent weight gain, 2) development of newly diagnosed type 2 diabetes, and 3) markers of insulin resistance/glucose metabolism abnormalities, including fasting insulin, hemoglobin-A1c, and total adiponectin, in young and middle-aged women who participated in a well-characterized prospective cohort: The Nurses' Health Study (NHS) II. We will also explore lifestyle and dietary determinants of the POPs and potential interactions between POPs and other risk factors on obesity and diabetes in this cohort. The NHS II provides a unique and ideal opportunity for us to examine the aims. Since the study baseline in 1991, extremely rich data on lifestyle, diet, medical history have been repeatedly collected every 2 to 4 years. Moreover, in 1994-1995, blood samples were provided by approximately 30,000 NHS II participants, who comprised the study population for the current investigation. Among these participants, we will conduct a nested case-control study of type 2 diabetes, which is expected to include 1,050 incident type 2 diabetes cases at age 45 or older and the equal number of matched controls. Rigorous quality control protocol and processes have been and will be continuously applied in the NHS II cohort to ensure the highest quality of research data. In addition, we have assembled an outstanding research team comprised of well-established researchers in multi-disciplinary fields including environmental health, nutrition, and biostatistics that will ensure the successful implementation of the project. In summary, the current investigation will provide novel and critical evidence that will not only greatly expand ou knowledge of the diabetogenic effects of POPs, but also inform public health policy-making to help prevent obesity and T2D.